Sleeping on your back?
This was a big concern for me in my first pregnancy, as I roll around a lot in my sleep and have very little control over what position I sleep in. Everyone I talked to about this, from midwives to academic OBs, said that if being on my back was comfortable for me and didn't make me feel dizzy, it was fine. I did find that as the pregnancy progressed, I was more comfortable on my side, preferably with a great big body pillow.
My doctor told me that I can. He also said that sleeping on your left side is no different than the right. He said that it is an old wives tale that you can't. IDK. I was sleeping with a Boppy body pillow,(which I bought because I thought we had to sleep on side) thinking it would be a healthier allignment but was waking up with my knees throbbing in pain......for weeks. I stopped using it and the pain is gone!!!!!! Amazing! I think we are all different. Back sleeping became uncomfortable for me weeks and weeks ago. Short of breath and dizzy.